In recent years, 100 Gigabit-per-second (Gbps) long-distance optical transmission has been implemented by dual-polarization quadrature phase-shift keying (DP-QPSK) using a digital coherent technology. To further improve transmission capacity, greater-level modulation schemes such as polarization division multiplexed 16 quadrature amplitude modulation (16-QAM) are being developed. Demand for downsizing optical transceivers is also increasing. At present, lithium-niobate (LiNbO3) Mach-Zehnder modulators are used typically as optical modulators. In order to realize downsized DP-QPSK or DP-16QAM transmitters, semiconductor Mach-Zehnder modulators are desired.
There is an intrinsic problem in semiconductor optical modulators in that the modulation characteristic (i.e., the relationship between applied voltage and amount of phase rotation, or the voltage-to-phase change characteristic) varies depending on the wavelength of a light beam input to the modulator. In semiconductor optical modulators, the absorption edge wavelength of the semiconductor material changes according to applied voltage, and the phase of light is modulated making use of the phase shift due to absorption based on Kramers-Kronig relations. Hence, semiconductor optical modulators have wavelength dependency such that the closer to the absorption-edge-wavelength the light to be modulated is, the greater the optical phase change with respect to the voltage change becomes.
On the other hand, because the absorption edge wavelength of a semiconductor optical modulator changes in response to a change in the substrate bias voltage, the modulation characteristic can be controlled. In this context, a “substrate bias voltage” is a direct-current (DC) bias voltage for controlling a modulator operating point (which voltage corresponds to a center voltage of a high-frequency electric signal for driving the optical modulator). The substrate bias voltage is distinguished from other types of bias voltages. Other types of bias voltages include an optical phase bias voltage for controlling a phase difference of light propagating through the two optical waveguides of a Mach-Zehnder interferometer, and a π/2 shift bias voltage for controlling the optical phase difference between two Mach-Zehnder interferometers to π/2 radians when performing orthogonal phase shift keying.
To address the wavelength dependency of the modulation characteristic of semiconductor optical modulators, several techniques for controlling a substrate bias voltage or amplitude of a drive signal according to the wavelength of input light are proposed. The first technique is to set the substrate bias voltage to a predetermined level according to the wavelength, and drive the modulator at a constant amplitude of a drive signal regardless of the wavelength. See, for example, Japanese Laid-open Patent Publication No. 2005-326548 A.
The second technique is to perform feedback control on the substrate bias voltage or drive signal amplitude. A low frequency signal is superimposed on driving data signals, and output light signals are monitored. Responsive to the monitoring result, the substrate bias voltage and/or the amplitude of the modulator drive signal is controlled. See, for example, Japanese Laid-open Patent Publication No. 2012-257164 A.
In semiconductor Mach-Zehnder modulators, the voltage-to-phase change characteristic (i.e., modulation characteristic) or the wavelength characteristic may vary between the two optical waveguides of the Mach-Zehnder interferometer. The two optical waveguides may undergo different changes with time in terms of the voltage-to-phase change characteristic. In addition, driving circuits (or driving amplitudes) for driving the optical waveguides may also be subjected to different changes with time. Even with a LiNbO3 Mach-Zehnder modulator that performs optical phase modulation making use of electrooptic effect, modulation amplitude may vary between the two optical waveguides of the interferometer, due to change with time in driving amplitude.
The conventional first and second techniques drive the two optical waveguides of the Mach-Zehnder interferometer using a driving waveform with the same amplitude and they apply the same substrate bias voltage to the two optical waveguides. Accordingly, when the voltage-to-phase change characteristic or driving amplitude varies between the two optical waveguides, or when the two optical waveguides undergo different changes with time, then the optical modulator is offset from the optimum modulation condition.
Accordingly, it is desired to provide an optical communication device and a technique for controlling an optical modulator that can maintain the optimum modulation condition even if the modulation characteristic varies between two optical waveguides of a Mach-Zehnder interferometer or the two optical waveguides undergo different changes in terms of the modulation characteristic.